A serious disadvantage of current dental X-ray machines employing rotational panoramic radiographic techniques for producing continuous image radiographs is that the resultant has a very narrow layer in the anterior region. Not only are the images excessively narrow at the anterior region, but the images thereat are often distorted and unsharp due to improper patient positioning while the anterior region is being radiographed.
The thin image layer at the anteriors produced by many current panoramic dental X-ray machines results from a very small effective projection radius at the anterior region during rotational scan thereof by the X-ray beams. If the projection radius at the anterior region could be increased without a concomitant distortion or inaccurate portrayal of the remaining dental arch and temporomandibular joint area, the layer at the anterior region could be widened and patient positioning would therefore not be so extremely critical.
We have discovered that the effective projection radius at the anterior region may be lengthened if the effective center of rotation of the X-ray beams are caused to retreat immediately after a canine is projected until completion of the radiographing of one-half the incisors, and the path of the effective centers of rotation of the X-ray beams then proceeds to trace a mirror image of the retreat path until the entire anterior region is radiographed. The retreat, or reversal, or backward movement of the effective centers of rotation of the beams within the dental arch toward the X-ray source may be achieved by controlling the movement of the patient chair and adjusting film travel speed accordingly, both later described.
As is known, rotational panoramic radiography may include employment of mechanisms for controlling film travel speed; orbiting the X-ray tubehead and camera assembly about the head of the patient; and shifting the patient chair in accordance with predetermined patterns. Each of these movements must be painstakingly integrated and synchronized if meaningful continuous radiographs are to be provided. Structure for providing such continuous image radiographs employing rotational panoramic radiographic techniques are shown and described in U.S. Pat. No. 4,251,730, assigned to the present assignee hereof. The subject matter of U.S. Pat. No. 4,251,730 is herein incorporated by reference and relates to an x-motion drive for moving or transporting the patient chair in a straight line, as the patient chair moves in the present invention, as opposed to x-y or non-straight line chair movement.
Briefly, the panoramic dental X-ray machine described in U.S. Pat. No. 4,251,730 may be slightly modified by any of a number of suitable means to provide the improved layer thickness at the anterior region. For example, in order to achieve the aforediscussed desired backward movement of the effective centers of rotation of the X-ray beams in accordance with one aspect of the present invention, the patient chair movement was altered by configuring the path of groove 256 in cam member 244 (U.S. Pat. No. 4,251,730) in accordance with readily calculable mathematical values. Grooving a different path for each cam for each new chair movement however is costly and time-consuming.
The desired chair movements may otherwise be achieved by suitable x-motion drives; y-motion drives; x-y motion drives; a stationary patient chair with an X-ray tubehead following a predetermined path; or combinations thereof. Preferably, however, a more versatile chair movement control includes a stepper motor controlled by microprocessor means wherein signals or impulses generated thereby are fed to the motor for transmitting desired motion to the chair.
Film travel speed is made proportional to the effective projection radius, i.e., the longer the effective projection radius, the faster the film should travel in the camera. Film drive mechanisms and structure associated therewith are shown and described in the following U.S. Pat. Nos., all assigned to the present assignee: 4,172,977; 4,194,121; 4,221,970; 4,247,779; 4,287,423; and 4,304,998.
Film travel speed is similarly preferably controlled by microprocessor means connected to a suitable stepper motor. Programming of microprocessors is well known for performing specified functions and is not herein shown or described.
The present invention is directed to and discloses methods for increasing the effective projection radius at the incisors or anterior region by providing a backward or reverse movement to the effective centers of rotation of the X-ray beams by means of controlling chair movement and film speed. Any of the suitable means abovementioned for providing the desired velocity profiles to the chair movement and film speed may be used. By thus increasing the effective projection radii at the anterior region, an improved wider layer thereat results.